The present disclosure relates to a multi-junction (also called a tandem-type, a stack-type, or a lamination-type) solar cell, a photoelectric conversion device, and a compound-semiconductor-layer lamination structure that use a compound semiconductor.
As a compound semiconductor configured of two or more types of elements, many types exist depending on a combination of the elements. Also, by laminating a lot of compound semiconductor layers made of different materials, a compound semiconductor device having various functions and various physical properties are achievable. As an example thereof, a solar cell may be mentioned. Here, as a solar cell, a silicon-based solar cell that uses silicon as a semiconductor, a compound semiconductor solar cell that uses a compound semiconductor, an organic solar cell that uses an organic material, etc. may be mentioned. In particular, the compound semiconductor solar cell has been developed aiming further improvement in energy conversion efficiency.
As a means for improving energy conversion efficiency of the compound semiconductor solar cell, there are provided a method in which a plurality of sub-cells each configured of a thin-film solar cell that is configured of a plurality of compound semiconductor layers are laminated to form a multi-junction solar cell, a method in which an effective combination of compound semiconductor materials configuring the compound semiconductor layers are searched, etc. Each of compound semiconductors such as GaAs and InP has a unique band gap, and a wavelength of light to be absorbed is different depending on this difference in band gap. Therefore, by laminating a plurality of types of sub-cells, efficiency of absorption of solar light that has a wide wavelength range is improved. In lamination, a combination of lattice constants and physical property values (such as band gaps) of crystal structures of the compound semiconductors configuring the respective sub-cells is important.
By the way, most of the multi-junction solar cells under current consideration are classified into a lattice-matched type and a lattice-mismatched type. In the lattice-matched type, compound semiconductor layers are laminated that are made of compound semiconductors having lattice constants that are almost the same with one another. In the lattice-mismatched type, compound semiconductor layers are laminated that are made of compound semiconductors having lattice constants that are different from one another with the use of metamorphic growth accompanied by dislocation. However, in the metamorphic growth method, undesirable lattice mismatch inevitably occurs, and therefore, there is an issue that quality of the compound semiconductors is significantly lowered.
On the other hand, in recent years, there has been proposed a method of manufacturing a multi-junction solar cell that utilizes a substrate bonding technique in junction of compound semiconductor layers, and a four-junction solar cell that has a structure of In0.48Ga0.52P/GaAs/InGaAsP/In0.53Ga0.47As has been reported.
This substrate bonding technique is a technique to form homojunction or heterojunction between the compound semiconductor layers to be joined, and may be classified, for example, into a direct bonding scheme in which different compound semiconductor layers are bonded directly to one another (for example, see Non-patent Literature 1: “Wafer Bonding and Layer Transfer Processes for High Efficiency Solar Cells”, NCPV and Solar Program Review Meeting 2003), and a scheme in which the compound semiconductor layers are joined with a connection layer in between. The substrate bonding technique has an advantage that it is not accompanied by an increase in threading dislocation. Existence of the threading dislocation leads to a not-preferable effect on electron performance of the compound semiconductor layers. In particular, the existence of the threading dislocation provides an easy diffusion path in the compound semiconductor layers as with a dopant and a recombination center, and causes a decrease in carrier density of the compound semiconductor layers. Also, the substrate bonding technique resolves the issue of lattice mismatch, and further avoids epitaxial growth caused by the lattice mismatch. Therefore, threading dislocation density that degrades the performance of the solar cell is largely reduced. In this substrate bonding technique, a covalent bonding is formed in an interface between different substances, in particular, in a hetero interface. At this time, it is important to perform a substrate junction process at a temperature by which thermal variation does not exceed a dynamic barrier necessary for progression in threading dislocation.
In junction by the direct bonding scheme, semiconductor-semiconductor bonding is performed in a nuclear scale. Therefore, transparency, heat conductivity, heat resistance, and reliability of the junction portion are superior than those in a case where junction is formed with the use of metal paste, a glass raw material (frit), etc. In this direct bonding scheme, an integrated-type or two-terminal compound semiconductor device is allowed to be integrated to a module with simplicity equivalent to that in a solar cell configured of a single-junction device, specifically, only by alloying the respective compound semiconductor layers to be laminated.